1. Field of the Invention
The present invention relates to an optical disk device, and more particularly to a technique for detecting a pre-pit formed on an optical disk.
2. Description of Related Art
In optical disks such as DVD-R, DVD-RW or the like, a pre-pit having address information regarding an information recording track is formed in a track adjacent to the information recording track, for example, in a land adjacent to a groove on the outer peripheral side. When recording and reproducing data, the address of the optical disk is specified by detection of the pre-pit.
Typically, a pre-pit is detected in the following manner. Specifically, a laser beam is irradiated toward the center of the information recording track and the return light therefrom is converted into an electrical signal by four segment photodetectors which are disposed in the radial and circumferential directions. Then, a difference between two signals obtained from two photodetectors which are disposed apart from each other in the radial direction is calculated. As a result of this difference calculation, a modulation component of recording power is removed, so that a pre-pit signal contained in the difference signal is extracted and decoded.
However, when a difference between two signals output from the two photodetectors which are disposed apart from each other in a radially divided manner as described above, it is not possible to completely remove a modulation component of recording power, leaving such a modulation component unremoved as noise, especially at edge portions immediately after start and completion of recording. In addition, since the laser power is greater in recording than in reproducing and an amount of return light is also increased in the recording power, the noise level for recording power can be increased to a level equal to or greater than the pre-pit level of the reproducing power. In such a case, it is not possible to distinguish noise of the recording power from a pre-pit signal of reproducing power with regard to their levels, which makes it difficult to extract the pre-pit signal of the reproducing power when the difference signal is binarized with a predetermined threshold value so as to extract the pre-pit signal. Namely, if the threshold value is increased to remove the noise of recording power, the pre-pit signal of the reproducing power is also removed. If the threshold is decreased to extract the pre-pit signal of the reproducing power, on the other hand, the noise in the recording power is also extracted erroneously as a pre-pit signal. Since the period for applying the recording power corresponds to a mark period whereas the period for applying the reproducing power corresponds to a space period, the foregoing problem can be regarded as a problem that it is difficult to extract a pre-pit signal during the space period.
While it is possible to overcome the foregoing problem using the structure as disclosed in Japanese Patent Laid-Open Publication No. 2000-311344, in which a circuit for detecting a pre-pit during a mark period and a circuit for detecting a pre-pit during a space period are provided in parallel, so that pre-pits are detected in the mark and space periods, respectively, this structure also has a problem. Specifically, this structure requires a circuit for generating a gate signal used for processing a signal in each of the mark and space periods, which makes the circuit configuration complicated.